The present invention relates to embodiments of an adaptive cruise control system capable of transmitting vehicle gear transition messages and brake control message during deceleration in case of inadequate retarder response or a retarder malfunction. When an adaptive cruise control system determines that vehicle deceleration is necessary to maintain a time to target, it will begin transmitting deceleration messages to vehicle subsystems, such as the retarder, the engine and the braking system, to assist with vehicle deceleration. If deceleration is not occurring at the rate expected by the adaptive cruise control system because the time to target keeps getting smaller, heavy braking may be requested. Heavy braking causes sudden g-force changes that can shock the vehicle operator. There is a need for an adaptive cruise control system that mitigates sudden g-forces when decelerating.